Cemented carbides for cutting tools, as composite materials of a WC hard phase and a cobalt (Co) binder metal phase, are representative dispersion alloys, wherein their mechanical properties depend on a particle diameter of the WC hard phase and an amount of the Co binder metal phase, hardness and toughness are particularly in an inversely proportional relationship to each other, properties required for the cemented carbides for cutting tools vary according to machining methods, and accordingly, various attempts have been made to control the mechanical properties of the cemented carbides.
Recently, in machining market, there is a growing demand for a shorter cycle time to improve competitiveness through cost reduction. In order to reduce the cycle time, since machining conditions are gradually changed to high-speed, high-feed conditions, there is an increasing need to allow physical properties of the corresponding cutting tool to have characteristics in which both wear resistance and toughness are good at the same time so that good machining may be performed even under the high-speed, high-feed conditions.
Accordingly, with respect to a hard coating formed on the cutting tool, a coating including an alpha-phase alumina layer, which has excellent stability at high temperature, has been preferred, and, with respect to a MT-TiCN layer formed as an underlayer of the alumina layer, a fine and uniform columnar structure has been preferred due to a trend towards high hardness.
In a case in which non-uniform plastic deformation occurs in a base material of the cutting tool, since chipping easily occurs in a high-hardness film formed on the base material, stability of base material characteristics in a direction perpendicular to the film is required to allow physical properties of the high-hardness film to be fully exhibited.
In a surface portion of the base material on which the hard film is formed, a toughness reinforcement layer (Cubic phase Free Layer, hereinafter, referred to as “CFL”), in which a cubic carbide constituting the base material is not present, is formed from a surface to a depth of about 10 μm to about 40 μm so as to absorb an impact generated during machining as disclosed in a patent document (Korean Patent Application Laid-open Publication No. 2005-0110822), wherein uniformity of the CFL (uniformity of microstructure by location, uniformity of composition by location) is required for the above-described high-hardness film.
However, with respect to a CFL of a current commercially available cemented carbide, there is a tendency that a Co structure toward a surface is small and an irregular coarse Co structure is formed as it moves to the inside of the CFL, and, since the irregularly formed coarse Co structure disturbs the uniformity of the CFL, it may be a cause of deteriorating overall physical properties of the cutting tool.
For this reason, as the current technological trends, development focusing on reducing a thickness of the CFL while using the high-hardness film are being conducted to improve wear resistance and plastic deformation resistance of the cutting tool. However, when the thickness of the CFL, which functions as the toughness reinforcement layer absorbing an external impact, is excessively decreased, the function of the CFL as an impact absorbing layer may be rapidly reduced to reduce the toughness of the cutting tool.